There are instances where large surfaces may become contaminated with various hazardous materials. For example, international hostilities or terrorist activities may provoke the dispersion of a chemical warfare agent over a broad area, covering a variety of surfaces. Other examples are industrial or transportation accidents in which a hazardous chemical may be spilled or leaked. A number of chemical substances which might be dispersed in such situations are very hazardous, and at the present time, there is no efficient means to remove such chemical substances from certain surfaces by conventional means. An attempt to remove such chemicals by washing would be relatively slow, provide little assurance of substantially complete removal, would spread the chemicals to other areas, e.g., through the washing solutions, and would pose a serious health hazard to workers removing the chemicals.
The present invention provides a method of photometrically removing contaminants from surfaces using apparatus that is readily transportable to the site where decontamination of surfaces is needed and which not only removes hazardous chemicals from surfaces, but substantially decomposes many such chemicals during removal. Decontamination by the method of the present invention is easily controllable, allowing chemicals to be removed from the surface without significant damage to the surface itself.
More specifically, the invention provides for removal of surface contaminants by exposure of a contaminated surface to pulses of high-intensity, polychromatic, incoherent light radiation, such as is provided by powerful xenon flashlamps, e.g., those sold under the tradename "Flashblast" by Maxwell Laboratories, Inc. It is known to remove materials from surfaces using pulses of such radiation. For example, removal of barnacles, etc. from the metal hulls of ships using high-intensity flashlamp radiation is described in T. Johnson, Popular Science (1982) pp. 82-84.
It is found, however, that merely applying flashlamp radiation to a surface that is contaminated with a thin film or layer of hazardous contaminant does not provide satisfactory removal of the contaminant. If the contaminant and the surface are each non-absorbing of broad-band frequently light, e.g., a colorless contaminant on a glass or clear plastic surface, there will be insufficient heat generated in the region of the surface to vaporize and/or decompose the contaminant. If a contaminant that is substantially non-absorbing of light covers a light-absorbing surface, the surface will absorb the major portion of the radiation, and the surface may be marred by the heat generated at the surface. Furthermore, if the heating takes place primarily at the surface itself, there is a tendency for any surface contaminant layer to be dispersed from the surface in droplets, in which case, the surface tends to become recontaminated by the droplets as they settle thereon. If contaminants are to be permanently removed from surfaces, therefore, it is desirable to preferentially heat the contaminant relative to the surface.
Japanese Pat. No. 119,181 to Nippon Steel Corp. describes a method of removing rolling lubricant from- steel plates. An IR-absorbing chemical is admixed with the lubricant that is applied to the steel as it is rolled. Subsequently, the surfaces of the steel plate are exposed to CO.sub.2 laser radiation which is substantially monochromatic light in the infrared (IR) frequency range, the IR-absorbing chemical heating to a temperature whereat the chemical and the lubricant are removed from the surface. While this system is applicable to a process where chemicals are removed at a fixed location, e.g., at a certain step in a steel rolling process, it is generally impractical for decontaminating surfaces at locations, however remote, where contaminated surfaces might be found. CO.sub.2 laser apparatus is expensive, delicate and difficult to transport, and would therefore be difficult to bring to the site where decontamination apparatus is needed. Furthermore, the combination of IR-absorbing chemicals with an IR-light source, is generally inapplicable to contaminant removal with pulsed broad-band radiation. Broad-band light that is weighted to IR frequencies may be produced by flashlamps in conjunction with a suitable window or lens material; however, due to internal heating problems, broad-band frequency light, heavily weighted to the IR frequencies can only be produced of relatively low power which is insufficient for vaporization and/or decomposition of surface contaminants.